AMD RX550 (4GB) GPU - OpenCL not available in Cycles Rendering Compute
I know this isn't the answer you're looking for (it's not one I'm happy with either), but, at the moment—even two months after you first asked…

You either need a different video card or a different OS.

OpenCL support is currently broken in the official AMD drivers for the RX 550 specifically {1}. This is still true as of the 17.30-465504 AMDGPU-PRO drivers and Ubuntu 16.04.3 (which they finally support).

While the card supports OpenCL natively and works in Windows, it is unclear when fully working (i.e. including OpenCL) Linux drivers will be released.

A comment on the AMD Community boards suggests that there is a working internal build that has been tested against the RX 550 as of August 11th, 2017. When this will be publicly released is unclear as of now. In addition to the evidence I've linked to supporting this conclusion, I've personally confirmed that the issue is specific to the RX 550 by dropping an RX 580 into a Ubuntu 16.04.3 system with identical drivers without any issue: the RX 580 shows up and works fine with OpenCL, the RX 550 continues to not be seen as an OpenCL device.

I've also personally tried the oibaf open source drivers, but have not been able to get OpenCL working on the RX 550 with them. This weekend I will give that another try using Ubuntu Zesty instead of the 16.04 LTS, now that I have an RX 580 to help act as a control comparison case.

I would suggest posting in the above linked thread or contacting AMD directly, referencing the linked comment, and asking for the internal build that is working for OpenCL on the RX 550. Otherwise the only available options seem to be either waiting for them to be released publicly, getting a different graphics card, or not using Linux with the RX 550 for now.

{1}: Supporting evidence (in addition to the thread linked in the second paragraph):


https://community.amd.com/thread/217469
https://github.com/ethereum-mining/ethminer/issues/88
https://community.amd.com/message/2804598